Driving SiC Junction Transistors (SJT)

Application Note AN-10A:
Driving SiC Junction Transistors (SJT) with
Off-the-Shelf Silicon IGBT Gate Drivers:
Single-Level Drive Concept
Introduction
GeneSiC Semiconductor is commercializing
1200 V and 1700 V SiC Junction Transistors (SJT)
with current ratings ranging from 4 A to 16 A. SiC
SJTs are normally-off, high-performance SiC
switches, which are plug-in replacements for Si
IGBTs. This document is the first of a two-part
application note which will describe simple, yet
optimized techniques for driving the SiC SJTs with
a commercially available IGBT gate driver. This
document describes a single-voltage level gate
drive scheme, whereas a more optimized twovoltage level gate drive scheme for the SiC SJTs is
described in GeneSiC document AN-10B.
SJT Gate Drive Circuit
A simplified gate drive schematic for driving
the SJT, based on a single-voltage level concept is
shown in Fig. 1. It features a commercial gate
driver IC with an isolated input signal and a
resistor-capacitor output network for improved
switching performance.
In comparison to SiC MOSFETs, which require
a non-standard +20 V gate bias due to poor
transconductance characteristics, the SJTs can be
driven with gate voltages as low as 8–10 V. The
SJT also does not require a negative gate voltage to
remain off.
The gate drive IC must be capable of supplying
a continuous current of ~500 mA to the SJT gate
during on-state operation. The external parallel
gate resistor, RGP should be adjusted to meet this
requirement. As will be described in this
document, the external parallel capacitor, CGP can
be appropriately chosen to ensure an optimum
level of dynamic gate current during turn-on and
turn-off initial transients. This dynamic current is
essential for fast charging of the SJT’s internal
gate-source capacitance. The presence of this
paralleled resistor and capacitor on the output of
the gate driver can increase device switching
speed, reduce device switching loss, and reduce
driver losses as well. The selection of these
component values is addressed later in this
document.
While the IXYS IXDN614 gate driver [3]
described in this document has shown to be
capable of driving numerous SJT models without
issue, several other commercially available driver
options exist and may be employed for driving SiC
SJTs. For the IXDN614, the output voltage VO
equals the supply voltage VGG during the “high”
output. Sufficient coupling capacitance (≥ 470 µF)
should be added to the driver IC supply terminals
to ensure consistent output power supply into the
+VGL
+VGL
100 nF
470 µF
100 nF
SiC SJT
D
Gate Control Signal
VCC
IN/A
Optocoupler/
Isolator
(ACPL-4800)
+5V / 0V
GND/K
VGG
OUT
OUT
Choke
IN
IG
VO
Gate Driver
(IXDN614)
RGP
GND
CGP
G
Choke
S
VEE
Figure 1 – SJT Single-Level Gate Drive Circuit
AN-10A May 2013
http://www.genesicsemi.com/index.php/references/notes
1
Dclamp
Lload
Cbank
VDS
D
Gate
Drive
G
DUT
S
Figure 2 – Standard double pulse switching test circuit.
Table I – Double Pulse Example Testing Conditions
Parameter
Value
DUT
1200V / 6A SJT (GA06JT12)
1200V / 5A Schottky (GB05SLT12)
Dclamp
VDS
600 V
ID
6A
Vgg*
15 V
RGP*
22 Ω
CGP*
18 nF
Lload
1.05 mH
T
25 °C
*- Denoted in Fig. 1
SJT.
Due to the high voltage capability of SiC SJTs,
an optocoupler or isolator should be used to protect
the input signal source from potential high drain
voltages. The isolation rating should greatly
exceed the predicted DC voltages in use,
particularly with an inductive load present. Also,
choke coils are shown to be effective in reducing
common-mode noise in the circuit on voltage
supplies and gate driver inputs and outputs, they
may be used when and where necessary.
SJT Switching Performance
An industry standard double-pulse switching
test, shown in Fig. 2, has been performed to
demonstrate SJT switching performance using the
described gate drive circuit. During testing, the
SJT is turned on with the application of a gate
current IG and the drain current ID is ramped up
AN-10A May 2013
Figure 3 – Turn-on (top) and turn-off (bottom) switching
waveforms of a 1200 V / 6 A SJT (GA06JT12-247).
linearly while flowing through the inductor and
SJT in series until ID = 6 A when the SJT is
switched off. The device is then switched back on
after a 2 µs delay to record device turn-on. Voltage
and current waveforms of the SJT are shown in
Fig. 3 of both SJT turn-on and turn-off [5].
Given the test conditions in Table I and
utilizing the Fig. 1 gate driver, the SJT has a drain
current rise time tr of only 16 ns and a fall time tf of
26 ns. The total device switching energy loss is
only 97 µJ per cycle, equating to less than 10 W of
device switching loss at 100 kHz. The SJT can be
switched faster or with lower losses based on the
values of RGP and CGP, these changes are detailed
in the following section.
Gate Drive Parameter Selection
Driving an SJT is simple and nearly identical to
driving a Si IGBT, as described previously. The
http://www.genesicsemi.com/index.php/references/notes
2
Figure 4 – Transient gate current IG waveform while driving a
1200 V / 6 A SJT.
Figure 6 – Effect of external gate capacitance CGP on peak gate
current IG,pk.
SJT gate during turn-on, forward steady state, and
turn-off, as shown in Fig. 4. Of these, the steady
state gate current IG,SS during forward steady state
is determined by RGP. The IG,SS value after turn-on
transients are complete is calculated by the
equation
, =
Figure 5 – Suggested RGP values for 1200 V, GAXXJTXX series
SJTs for VO = 15 V.
presence of the parallel resistor and capacitor on
the gate driver IC output produces a dynamic gate
current waveform due to the presence of a transient
gate current peak from the charging of CGP which
turns the SJT on and off more quickly and also
reduces device losses. An example of this can be
seen in the Fig. 4 gate current waveform. Adjusting
the gate resistor RGP, capacitor CGP, and gate driver
output voltage VO will alter the static and dynamic
performance of the SJT with a tradeoff of
switching speed to device and driver losses to fit
the particular applications demands.
Gate Resistance RGP
The gate resistor RGP is an external resistor in
parallel with an external gate capacitor CGP. RGP
and CGP dictate the gate current entering into the
AN-10A May 2013
−,
,
(1)
in which VGS,on ≈ 4 V. RGP must be chosen in
tandem with VO to satisfy the IG drive requirements
as shown on the SJT device datasheet for the
desired drain current ID. For optimal SJT
performance in many applications, it is suggest to
overdrive the SJT by supplying more gate current
than absolutely necessary to ensure operation
across all rated temperatures with a low VDS value.
This can be considered as having a lower effect
current gain or “forced” current gain βF in the
range of approximately 12 < βF < 15 for
GAXXJTXX series SJTs. RGP and IG should be as
low as practical given this consideration and
approximate RGP values for a range of 1200 V SJT
current rating is shown in Fig. 5. RGP = 22 Ω has
shown good results for VGG = 15 V.
Gate Capacitance CGP
The presence of CGP produces a transient gate
current peak which speeds up the charging of the
internal SJT gate capacitance and increases SJT
switching speeds. For a fixed driver output voltage
VO, higher capacitance will cause larger current
peaks as shown in Fig. 6 where CGP values from 0
http://www.genesicsemi.com/index.php/references/notes
3
Figure 7 – Effect of external gate capacitance CGP on device turnon tr and turn-off tf times.
Figure 9 – Effect of gate capacitance CGP on device energy losses.
TABLE II – OPTIMAL DRIVE PARAMETERS FOR 1200 V/6 A SJT
Parameter
VO, VGG
RGP
CGP
Suggested Value
15 V
22 Ω
9 nF
while still obtaining desired switching speeds.
Ringing may occur in the gate drive output
network due to CGP, and the parasitic inductances
in the gate drive circuit. To reduce this, a low
inductance resistor of 1 Ω may be placed in series
with CGP to damp this oscillation. CGP = 9 nF is a
suggested value in most applications.
Figure 8 – Effect of gate capacitance CGP on device energy losses.
nF (only RGP present) to 100 nF are compared.
Higher gate current peaks correlate with faster ID
rise and fall times in the SJT, particularly fall
times, as shown in Fig. 7, however the speeds
saturate or decrease for CGP > 18 nF.
The value of CGP also affects device losses Eon
and Eoff. This is shown in Fig. 8, in which SJT
device losses are lowest for CGP = 9 nF. Any
increase or decrease in capacitance value causes a
sudden or subtle change in device loss.
Excessive capacitance also increases driver
losses due to the power required to charge CGP as
shown by the equation
drive,sw = ( − )2 .
(2)
Therefore, CGP should be set to be as low as
practical to minimize both device and driver losses
AN-10A May 2013
Gate Driver Voltage VO
The gate driver output voltage VO also affects
SJT performance and is adjustable. VO must be
sufficiently high to bias the SJT gate-source
junction on, which has a built-in voltage of
~ 2.8 V, and also to supply the steady state gate
current IG,SS, after the gate current peak, according
to equation (1). In Fig. 9, it is shown that there is a
nearly linear decrease in Etot, tr, and tf with
increasing VO for fixed CGP and RGP values. Thus
there is no tradeoff of device switching
performance for higher VO, unlike CGP. However,
VO is known to be a large contributor to gate driver
loss in Equation 2 as well as steady state driver
losses (detail in document AN-10B), thus there are
drawbacks to excessive VO. Circuit designers
should balance their requirements for device
switching speed with driver power losses.
VO = 15 V is an appropriate value for 1200 V SJTs
in most applications.
http://www.genesicsemi.com/index.php/references/notes
4
TABLE III – GATE DRIVE POWER LOSS EXAMPLE
D = 0.7, f0 = 500 kHz
Parameter
Power Loss
3.85 W
,
0.54 W
,
45.6 W
,
Total
50.0 W
Fig. 10, where system losses increase linearly with
higher switching frequency.
Summary
SJTs are high-performance SiC switches
capable of fast switching speeds with ultra-low
losses without the drawbacks of other SiC
transistors or bipolar Si devices. A simple gate
drive schematic has been presented along with
resultant device switching performance. More
detailed considerations of passive component value
selection is also discussed with benefits and
drawbacks explained. More advanced SJT gate
drive schematics are discussed in document
AN-10B and future GeneSiC Semiconductor
application notes.
References
Figure 10 – Gate driver dependent system power loss as a function
of frequency for a fixed duty cycle of D = 0.7. Note that the SJT
conduction loss component is not considered here.
Power Loss
When the single-level gate driver is utilized
with optimal drive parameters to drive a 1200 V /
6 A SJT, the system power losses components in
both the gate drive and SJT which are dependent
on the gate driver topology are provided in
Table III. In this example case the switching
frequency is f = 500 kHz and D = 0.7, which is
within the capabilities of the SJT and the driver.
The steady state losses of the driver Pdrive,SS are
primarily duty cycle dependant while the switching
losses of both (Pdrive,sw and PSJT,sw) are frequency
and driver dependant. PSJT,sw begins to dominate
the system power loss for f > ~ 70 kHz as shown in
AN-10A May 2013
[1] D. Veereddy, S. Sundaresan, S. Jeliazkov, M.
Digangi, and R. Singh, “Breakthrough High
Temperature Electrical Performance of SiC
‘Super’ Junction Transistors,” Bodo´s Power
Systems, pp. 36–38, Oct-2011.
[2] S. Sundaresan, M. Digangi, and R. Singh, “SiC
‘Super’
Junction
Transistors
Offer
Breakthrough High Temp Performance,” Power
Electronics Technology, pp. 21–24, Nov-2011.
[3] “IXD_614 Low-Side Driver Datasheet.” IXYS
Inc.
http://www.ixysic.com/home/pdfs.nsf/www/IX
D_614.pdf/$file/IXD_614.pdf
[4] J. Rabkowski, G. Tolstoy, D. Peftitsis, and H.
Nee, “Low-Loss High-Performance Base-Drive
Unit for SiC BJTs,” Power Electronics, IEEE
Transactions on, vol. 27, no. 5, pp. 2633–2643,
2012.
[5] “GA06JT12-247 Datasheet.”
GeneSiC
Semiconductor
Inc.
http://www.genesicsemi.com
/index.php/sicproducts/SJT/
http://www.genesicsemi.com/index.php/references/notes
5